Right at the bridge over the river Riviere Langevin close to the village of Langevin there is a small road at the west side of the river leading to Grand Galet and into the river valley. After roughly 6 km you reach the first waterfall of the valley, Cascade du Trou Noir. There is a short side trail (10 min) to the falls which are located in a beautiful setting so take your lunch with you. To reach the second waterfall you really have to trust your driving abilities and your car since from now on the road gets very narrow and winds through extremely steep serpentines with narrow twists. In Europe, this road would have been designated as one-way road...on La Reunion you'll have two-way traffic on the same road. This means you have to be very careful while driving, never stop on your way UP (it's then hardly possible to get your car moving UP again) and blow your horn frequently. It's a real experience and after a short while it's even fun!
After the first serpentines you'll reach the second waterfall, Cascade de la Grand Ravine, located right at the roadside. The following serpentines only lead to the small village of Grand Galet where the road ends. So if you don't intend to hike further into the valley it makes no sense to drive beyond the second waterfall (unless you have fun driving, of course).